During the operation of electric power equipment in electric power substations and in switchgears that contain vacuum circuit breakers, in the course of the operation of switching the circuit breakers on and off, electric power equipment is exposed to very fast transients which are dangerous to the operated equipment. By way of example, very fast transients are generated when transformers are switched on or off by means of vacuum circuit breakers. Transformers are often connected with circuit breakers by means of cables of a length of several dozen or several hundred meters. Low value of impedance and insignificant cable loss cause that the amplitude of the generated very fast transients magnified by wave reflections at the connection points, which can considerably exceed the rated value of the supply voltage, and the frequency of such transients can range from a few hundred kHz to as much as a few MHz. Then very fast transients can damage the transformer insulation or its windings. A voltage of a short risetime of a few dozen or a few hundred kV/ps and oscillations of frequencies ranging from several hundred kHz to many MHz that accumulate on the transformer winding degrade the insulation and, in consequence, lead to its breakdown and internal faults. Therefore there is a need to eliminate or reduce the damaging effect of very fast transients by using an additional protective component or device. Typically, varistor surge arrester, surge capacitors of capacitances in the order of a few hundred nF, RC filters and pre-insertion resistors, connected in parallel with the circuit breaker contacts are used as additional components or devices against the damaging occurrence of very fast transients.
The use of varistor surge arresters ensures great efficiency of transient amplitude reduction, but it does not change the rate of rise of the voltage wave. Moreover, due to the character of operation of this type of suppressors, additional high-frequency voltage components are generated.
Solutions based on surge capacitors characterized by large capacitance value and R-C filters are efficient, but these have large dimensions and weights which make placing them in a common housing with the protected device or circuit breaker fairly difficult. In addition, although R-C filters provide good protection against large amplitude interference, the rate of rise of the first voltage wave is in many cases not reduced, which significantly affects the level of protection of the protected facilities. It takes place particularly when the connection between the breaker and the protected equipment is relatively short. Then, pre-insertion resistances connected in parallel with the system of the circuit breaker contacts are difficult to install and they require additional contacts. Moreover, the large power emitted on such a resistor and problems connected with its dissipating are by no means insignificant.
Another solution used to reduce very fast transients are devices in the form of R-L reactors connected in series and having specially selected parameters. They act as a conductor of minute resistance for the low frequencies of the applied voltage and as an additional series impedance for higher frequencies that occur during connection phenomena. These devises are characterized by low voltage drop on their own impedance and by a small value of power dissipated during operation in stable condition. However, although these devices are very effective in suppressing very fast transients, they have a certain inconvenient feature, namely their dimensions depend on the value of current flowing through these devices, and the significant power dissipated during the flow of large-value fault current can result in their thermal destruction. The above mentioned inconveniences prevent the use of this type of solutions for installation in switchgears located in electric power substations connected with wind power plants by means of power cables.
All the presented solutions either fail to ensure full protection, as is the case of varistor transient suppressors, or the presence of these devices in the power network during normal operating conditions, for the operating frequency of 50/60 Hz, causes the dissipation of additional power in these devices. For that reason, the presented solutions are not acceptable as devices that fully protect transformers working in power substations or in wind power plants. The inconvenient integration of such devices with switchgears and wind power plants prevents their use in such cases due to the limited space available in the nacelle gondola or in the tower of a wind generator.
Patent description U.S. Pat. No. 6,642,806 reveals a method that allows a reduction in the frequency of occurrence of transients and/or in the amplitude value, which consists in placing a magnetic core of high magnetic permeability around a lead that conducts electric current. The use of the magnetic core allows to reduce the dimensions of the device which protects equipment against transients. If the device according to the presented solution is used, the efficiency can be insufficient due to the limited efficiency of the suppression of potential oscillations of transients only by the lossiness of the magnetic material of the core.
An additional disadvantage of this type of solution is the saturation of the core and thereby loss of the functionality of the device before the process of generation of very fast transients ends.
Patent description WO 2008/040128 reveals a method which allows to reduce the values of very fast transients, based on cores of a magnetic material arranged around a piece of a current-conducting lead and a resistor that shunts the piece of the current-conducting lead. An inconvenience of this solution is the need to make a galvanic connection between the shunting resistor and the current path, which requires a considerable modification of the current path.
Patent description of GB1187410 reveals an arrangement counteracting voltage surges due to circuit breaker interruption. In the arrangement electrically conducting lead is connecting with an interrupting contact of the circuit breaker. The conducting lead is encircled by a separate core of ferromagnetic material and in combination with the core is constituting a high frequency inductance device series connected with the interrupting contact. The core carries a secondary winding across which a resistor is connected.